Ultra-sensitive wideband diffraction-grating-coupled-mode Fabry-Pérot resonators for ultrasound and photoacoustic detection.

Optical ultrasound detectors offer broad bandwidth, high sensitivity, significantly reduced electrical and acoustic impedance mismatch, and seamless integration with transparent imaging platforms for biomedical and industrial ultrasound and photoacoustic applications. A diffraction-grating-coupled-mode Fabry-Pérot (GFP) resonator that enables wideband optical detection of ultrasound and photoacoustic signals employing a simple, low-cost fabrication flow is described. A 1D grating couples light between normal incidence and ±1st orders that are confined by total-internal-reflection boundaries, forming an effective FP cavity as a coupled resonator operating in Fano resonance. For illustration, GFPs with 2.1-µm, 4.9-µm and 10.1-µm cavities have been fabricated and comprehensively characterized. The scalable design achieved a maximum calibrated bandwidth >130 MHz and a noise-equivalent pressure of ∼72 Pa over 100 MHz. By combining strong optical confinement, favorable acoustic impedance matching, and scalable fabrication, GFP offers a practical solution to advance photoacoustic and ultrasonic sensing with a tunable responsivity-bandwidth trade-off for multimodal biomedical imaging.